Filter support for Docker

Fix #1110

gns3server/controller/link.py

@@ -365,7 +365,7 @@ class Link:
         :returns: None if no node support filtering else the node
         """
         for node in self._nodes:
-            if node["node"].node_type in ('vpcs', 'dynamips', 'qemu'):
+            if node["node"].node_type in ('vpcs', 'dynamips', 'qemu', 'docker'):
                 return node["node"]
         return None